Discharge lamp lighting device

Details Discharge lamp lighting device Discharge lamp lighting device

2001-12-13

2003-04-15

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S2090SC

Reexamination Certificate

active

06548966

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention is related to a discharge lamp lighting device which is used as the light source for an automobile headlight.
2. Description of Related Art
FIG. 13
is a circuit diagram showing a conventional discharge lamp lighting device, and in the figure,
1
is a DC power source such as a battery,
2
is a booster circuit for boosting the voltage of the DC power source
1
, and in the booster circuit
2
,
2
a
is a primary winding of a transformer connected to the DC power source
1
,
2
b
is a secondary winding for outputting the boosted voltage,
2
c
is a switching element connected to the DC power source
1
and the primary winding
2
a
,
2
b
is a diode connected to the secondary winding
2
b
, and
2
e
is a capacitor. Further,
3
is an earth line, and
4
is a current detection resistor for detecting the output current of the booster circuit
2
.
5
is a voltage detection circuit for detecting the voltage Va boosted to the minus side by the booster circuit
2
, and in the voltage detection circuit
5
, VDD is a 5 V power source, and
5
a
and
5
b
are resistors.
6
is an inverter circuit, in which
6
a
-
6
d
are switching elements.
7
is a control unit for controlling the switching element
2
c
and a driver
8
which is used for driving the inverter circuit
6
.
9
is a startup discharge circuit, in which
9
a
is a transformer,
9
b
is a discharge tube switch, and
9
c
is a capacitor. Further,
10
is a discharge lamp, in which
10
a
and
10
b
are electrodes.
FIG. 14
is a circuit diagram showing the details of the conventional control unit, and the control unit
7
in
FIG. 13
includes control circuits
7
a
,
7
b
, and
7
c
, an interface
7
d
(hereinafter abbreviated as I/F
7
d
), and a microcomputer
7
e
in FIG.
14
. Further, lines A to E in
FIG. 13
are connected to A to E in FIG.
14
.
In the figure, the control circuit
7
a
includes an operational amplifier B
1
, the power source VDD, resistors R
1
to R
3
, and a diode D
2
, the control circuit
7
b
includes a comparator A
2
, the power source VDD, a power source VCC (8 V power source), resistors r
3
to r
7
, and capacitors C
1
and C
2
, and the control circuit
7
c
includes a diode D
1
, an operational amplifier A
3
, a resistor r
8
, and a capacitor C
3
.
Now, the operation of the discharge lamp lighting device as configured above will now be described below.
In
FIG. 13
, when the voltage of the DC power source
1
is input to the booster circuit
2
and simultaneously the control unit
7
starts to operate, the control unit
7
outputs a pulse signal having a certain frequency and a certain duty ratio value to the gate of the switching element
2
c
, thereby to turn on and off the switching element
2
c
. Further, the control unit
7
also sends a signal to the driver circuit
8
. Whereupon, the driver circuit
8
sends a signal to the gates of the switching elements
6
a
and
6
d
to turn on and off the switching elements
6
a
and
6
d
. The duty ratio of the pulse signal sent to the gate of the switching element
2
c
can be varied by the control of the control unit
7
.
During the period over which the switching element
2
c
is ON, the current from the DC power source
1
is supplied to the primary winding
2
a
and electromagnetic energy is accumulated in the primary winding
2
a
. Whereupon, an induced counter-electromotive force occurs in the secondary winding
2
b
, but a reverse bias is applied to the diode
2
d
, and no current flows in the secondary side of the booster circuit
2
. During the period over which the switching element
2
c
is OFF, an induced counter-electromotive force occurs in the secondary winding
2
b
, a forward bias is applied to the diode
2
d
, and a loop is formed by the secondary winding
2
b
, diode
2
d
, and the capacitor
2
e
, so that the electromagnetic energy accumulated in the primary winding
2
a
during the ON period is accumulated in the capacitor
2
e
as electrostatic energy through the diode
2
d
. A voltage Va corresponding to that occurs in the secondary side of the booster circuit
2
. ON-OFF of the switching element
2
c
is repeated by the pulse waveform from the control unit
7
, and the voltage Va is gradually boosted to the minus side.
The voltage Va is input to the voltage detection circuit
5
, and it is divided by the resistors
5
a
to
5
d
and input to the control unit
7
. The control unit
7
continues to output to the gate of the switching element
2
c
a pulse signal of a fixed cycle, which has a predetermined value, for instance, a certain duty ratio value, and which is held when the voltage Va reaches −480 V. To hold −480 V of the voltage Va until the discharge lamp
10
starts to light, the duty ratio is controlled to be a small value of the order of 10 to 50% in the control circuit
7
a
in FIG.
14
. At this point, the output voltage of the operational amplifier B
1
lowers, and the diode D
2
is electrically conducted and draws out the electric charge of the capacitor C
1
to drop the duty ratio to about 10 to 50%. Further, the diode D
1
is not conductive immediately before and after the startup of lighting. In addition, since the switching elements
6
a
and
6
d
are ON, substantially the voltage Va is applied between both electrodes of the discharge lamp
10
.
After the elapse of some time since the voltage Va has reached −480 V, when the voltage difference between both electrodes of the discharge tube switch
9
b
of the startup discharge circuit
9
, or the voltage difference of the capacitor
9
c
reaches, for instance, about 400 V, the discharge tube switch
9
b
is turned ON and a current flows through the primary winding of the transformer
9
a
to generate a high-voltage pulse of about 20 kV in the secondary winding, and the high-voltage pulse is applied to the discharge lamp
10
to cause breakdown between both electrodes of the discharge lamp
10
, and a current flows through the discharge lamp
10
, which starts to light.
Whereupon, the voltage Va of the electrode
10
b
of the discharge lamp
10
rapidly rises from −480 V, and the voltage of the electrode
10
a
becomes a value obtained by multiplying the current flowing through the discharge lamp
10
by the value of the current detection resistor
4
. The rapid increase in the voltage Va is detected by the voltage detection circuit
5
, and the divided value of the voltage Va is sent to the control unit
7
. At this point, the control unit
7
detects the success of the startup discharge of the discharge lamp
10
, and continues to turn ON the switching elements
6
a
and
6
d
until about several tens msec after the discharge lamp
10
starts to light. With the rapid increase in the value of the voltage Va just after the breakdown of the discharge lamp
10
, the electrostatic energy accumulated in the capacitor
2
e
is supplied as a current to the discharge lamp
10
for about several tens to several hundreds &mgr;sec (discharge growth period). Thereafter, by the ON-OFF operation of the switching element
2
c
, electrostatic energy is again supplied to the capacitor
2
e
, and a current is also supplied to the discharge lamp
10
. Right after the breakdown, the output of the operational amplifier B
1
also increases as the voltage Va increases, the diode D
2
is not conducted, and the capacitor C
1
starts recharged, and thereafter the duty ratio is determined under the control of the control circuit
7
c.
The switching elements
6
a
and
6
d
are in the ON state for about several tens msec after the lighting startup of the discharge lamp
10
, applying a DC voltage to the discharge lamp
10
to stabilize the discharge, and thereafter a signal is sent from the control unit
7
to an input terminal of the driver circuit
8
, and a signal is then output from the driver circuit
8
for turning ON the switching elements
6
b
and
6
c
, thereby causing a current of the reverse direction to flow through the discharge lamp
10
for about several tens msec. There

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